1. Field of the Invention
This invention relates to a semiconductor device including trench-gate vertical MOSFETs (metal oxide semiconductor field effect transistors), and a DC-DC converter using this semiconductor device as a switching device.
2. Background Art
Recently, the demand for power MOSFETs has been rapidly increasing not only in the market for switching power supplies operating at high current and high breakdown voltage, but also in the market for energy-saving switching power supplies used in, for example, mobile communication devices such as notebook personal computers. The power MOSFET is used in a power management circuit and a safety circuit for a lithium-ion battery. Hence, the power MOSFET is desired to operate at lower driving voltage and lower on-resistance so that it can be directly driven by battery voltage. Furthermore, to reduce switching loss, reduction of gate-drain capacitance is desired.
In a typical configuration of the power MOSFET, an n−-type epitaxial layer is provided as a buffer layer on an n+-type semiconductor substrate connected to the drain electrode, a p-type base layer is provided on the n−-type epitaxial layer, an n+-type source region is formed partly in an upper portion of the p-type base layer, and a source electrode is connected to the n+-type source region. This configuration further includes a trench extending from the n+-type source region side, penetrating through the n+-type source region and the p-type base layer, and reaching an upper portion of the n−-type epitaxial layer. A gate dielectric film is formed on the inner surface of this trench, and a trench gate electrode is buried inside this trench (see, e.g., JP-A-2005-524970 (Kokai)).
However, in this type of power MOSFET, due to the stray inductance of the circuit including this power MOSFET, the source-drain voltage jumps at turn-off and exceeds the breakdown voltage, causing a large loss. This loss can be reduced by providing a capacitor (snubber circuit) between the source and the drain. However, providing such a capacitor increases the number of components. Furthermore, stray inductance occurring between the power MOSFET and the capacitor prevents the current from sufficiently flowing through the snubber circuit and results in the problem of low effectiveness.